Powder mixture

ABSTRACT

A powder mixture of the present invention is characterized in that it comprises: an iron powder or an iron-based alloy powder, wherein a graphite powder is adhered to a surface of the iron powder or the iron-based alloy powder by a binding agent containing a polyolefin wax; and a negatively-charged powder to be mixed with the iron powder or the iron-based alloy powder, and consisting of an iron powder and/or an iron-based alloy powder treated for negatively charging. This enables keeping the good adhering property of a graphite powder in a mixed powder and high flowability as a mixture of the raw material powder.

TECHNICAL FIELD

The present invention relates to a powder mixture used as a raw material powder in a powder metallurgy process, particularly in a die pressing method. In more detail, the present invention relates to the powder mixture in which the powder is prevented from segregation and the flowability of mixed powder is excellent.

BACKGROUND ART

As illustrated in FIG. 4, manufacture of a sintered machine part by the powder metallurgy process is performed by filling with raw material powder the cavity formed by a die hole 11 of a die 10 and a lower punch 20, followed by compacting the raw material powder between the lower punch 20 and a upper punch 30 to form a compact (so called the die pressing method) and heating to sinter the compact obtained in a sintering furnace. Such a die pressing method has an advantage of not only forming the sintered machine part in a near net shape but also allowing mass production of products with the same shape after a die for pressing is once manufactured, thereby lowering the manufacture cost. Therefore, the die pressing method is utilized in various fields.

The raw material powder used in the die pressing method is prepared by selecting a main raw material according to the properties desired for a machine part targeted and by adding an auxiliary raw material powder to the main raw material powder to mix. For example, in a machine part for structure the raw material powder is prepared by using as the main raw material powder an iron powder or an iron-based alloy powder, and adding graphite powder and an auxiliary raw material powder such as copper powder, nickel powder or a small amount of iron-based alloy powder with different component from the main raw material powder as needed, as well as adding a lubricant for compacting such as zinc stearate and mixing them. In the raw material powder, a machinability improving powder such as magnesium silicate type mineral powder or sulfide powder is also used as the auxiliary raw material powder as needed in order to improve the machinability of machine parts.

Incidentally, the raw material powder is generally stored in a hopper 40, from which the powder is conveyed by free fall under gravity through a hose 50 into a feeder 60 with the open bottom. As the feeder 60 moves over the die 10 to the position above the cavity formed of the die hole 11 and the lower punch 20, the cavity is filled with the raw material powder from the open bottom of the feeder 60. Therefore, the raw material powder requires the high flowability in order to keep smooth filling properties as well as to prevent variation in the amount of powder to be filled.

As described above, the raw material powder is prepared by adding the graphite powder and the auxiliary raw material powder as needed to the main raw material powder and mixing them so that the powder mixture is constituted with the powders different in size, shape, and specific gravity. As segregation of the graphite powder and the auxiliary raw material powder occurs, the powder has variation in composition causing change of dimension and large variation in properties such as strength and the like to yield defective products. Therefore, it is desired to prevent the graphite powder and the auxiliary raw material powder from segregation. Particularly since the density of graphite powder is smaller than the density of the iron powder or the iron-based alloy powder as the main raw material powder, the graphite powder with a smaller specific gravity is likely to scatter upwards when the powder slides in the hopper 40 caused by filling, so that it is strongly required to prevent the graphite powder from segregation.

With regard to such a problem of segregation various proposals are offered such that segregation is prevented by melting the lubricant for compacting to adhere the graphite powder and the auxiliary raw material powder to the surface of the main raw material powder (such as Patent Literature 1) or by adding a binder component to adhere the graphite powder and the auxiliary raw material powder to the surface of the main raw material powder (such as Patent Literature 2).

CITATION LIST Patent Literature

-   [Patent Literature 1] JP Hei01-219101 A -   [Patent Literature 2] JP Hei02-217403 A

SUMMARY Technical Problem

In various proposals above graphite powder and the auxiliary raw material powder such as copper powder, nickel powder, and the like is not sufficiently adhered to the surface of the main raw material powder, and even when the graphite powder and the auxiliary raw material powder are sufficiently adhered to the surface of the main raw material powder, the flowability of the raw material powder is reduced. Therefore, it is desired to develop the raw material powder for powder metallurgy, in which the graphite powder and the auxiliary raw material powder are sufficiently adhered to the surface of the main raw material powder and the flowability of the raw material powder is high. From such a background the purpose of the present invention is to provide a powder mixture in which segregation of graphite powder is prevented and the powder properties such as flowability are excellent.

Solution to Problem

In order to solve the problem above the present inventors carried out extensive study and found that the adhesive property is excellent in the powder obtained as follows.

(1) First, in order to adhere a graphite powder and the auxiliary raw material powder to the surface of the main raw material powder, a binding agent (polyolefin wax) and the main raw material powder are agitated and temperature is elevated to the melting point or higher of the polyolefin wax to coat the surface of the main raw material powder with the melted polyolefin wax. (2) Next, powder is produced by adding the graphite powder and the auxiliary raw material powder and lowering temperature to the melting point or lower of the polyolefin wax while agitating to adhere the graphite powder and the auxiliary raw material powder to the surface of the main raw material powder through the polyolefin wax.

However, the flowability of the raw material powder in which the graphite powder and the auxiliary raw material powder are adhered to the surface of the main raw material powder by the melting and mixing method above is reduced. The present inventors investigated a cause of reduction of the flowability and obtained the findings as follows.

That is, as schematically illustrated in FIG. 3 binding agent 4 is delaminated from parts of the surface of main raw material powder 1, graphite powder 2, and auxiliary raw material powder 3 in some of powder when agitated and mixed, making it difficult to completely coat the main raw material powder 1 with the binding agent 4. Incidentally, as illustrated in FIG. 4 while the cavity is filled with the raw material powder fed from the hopper 40 through the hose 50 and the feeder 60, the raw material powder flows in contact and friction with each other therewhile as well as parts of the raw material powder are conveyed in contact and friction with the inner wall of the hopper 40, the inner surface of the hose, and the inner wall of the feeder.

FIG. 2 illustrates the state of the electrification by friction thereat (Each letter and numeral is same as those in FIG. 3). Since the polyolefin wax contained in the binding agent 4 is a substance which is likely to be negatively charged and has electrical insulation properties, the polyolefin wax is negatively charged on the surface thereof by friction when conveyed. Copper powder used as the auxiliary raw material powder is also a substance which is likely to be negatively charged as well as binded in the electrically insulated state to the iron powder or the iron-based alloy powder through the polyolefin wax with electrical insulation properties so that the surface of copper powder exposed at the top is negatively charged by friction when conveyed.

On the other hand, the iron powder or the iron-based alloy powder as the main raw material powder is likely to be positively charged by friction when conveyed and the exposed surface is positively charged by friction when conveyed. As illustrated in FIG. 2( a) it is considered that the surface of the negatively charged polyolefin wax of other powder or the surface of the negatively charged copper powder are electrically attracted to the exposed part of the positively charged iron powder or the iron-based alloy powder to agglutinate, thereby reducing the flowability.

Then, the present inventors consider that as illustrated in FIG. 2( b) addition of the negatively-charged powder 5 with a high negative charge causes selective attraction of the negatively-charged powder with a high negative charge to the exposed part of the positively charged iron powder or iron-based alloy powder preventing other powder from attraction and preventing agglutination as well as further improving the flowability of the raw material powder by a repulsive force between the negatively-charged powder with a high negative charge remained without electrical attraction and each powder, and complete the present invention by generating this effect in the test.

A powder mixture of the present invention based on the findings above is characterized in that a negatively-charged powder consisting of an iron powder and/or an iron-based alloy powder treated for negatively charging gets mixed (addition and mixing) with the iron powder or the iron-based alloy powder, wherein a graphite powder is adhered to the surface of the iron powder or the iron-based alloy powder through a binding agent containing a polyolefin wax.

When the powder mixture comprises an auxiliary raw material powder, the powder mixture is characterized in that a negatively-charged powder consisting of at least one selected from the group consisting of an iron powder, an iron-based alloy, and an auxiliary raw material powder treated for negatively charging gets mixed (addition and mixing) with the iron powder or the iron-based alloy powder, wherein the graphite powder and the auxiliary raw material powder are adhered to the surface of the iron powder or the iron-based alloy powder through a binding agent containing a polyolefin wax.

A preferred aspect of the powder mixture in each of the present invention is that the polyolefin wax is at least one selected from the group consisting of polyethylene wax and polypropylene wax.

A preferred aspect of the powder mixture in each of the present invention is that the treatment for negatively charging is the coating treatment with at least one selected from the group consisting of an alkylsilane, dimethylsilane, octylsilane, methacrylsilane, fluoroalkylsilane, and hexamethyldisilazane. Further, a preferred aspect is that the maximum particle diameter of the negatively-charged powder is within the range from 1 to 15 μm, and an additive amount of the negatively-charged powder is from 0.02 to 0.5 parts by mass relative to 100 parts by mass of the iron powder or the iron-based alloy powder.

A preferred aspect of the powder mixture in each of the present invention is that when an auxiliary raw material powder is comprised, the auxiliary raw material powder is adhered through the binding agent to the surface of the iron powder or the iron-based alloy powder together with the graphite powder, but an aspect in which parts or all of the auxiliary raw material powder are contained in a form of free powder is valid. Also, a preferred aspect of the powder mixture in each of the present invention is that when a lubricant powder for compacting is comprised, the lubricant powder for compacting is contained in a form of free powder.

Advantageous Effects of Invention

Since in the present invention the powder mixture is a mixture wherein the graphite powder is adhered to the surface of the main raw material powder by using the binding agent containing a polyolefin wax with excellent adhering properties, segregation of graphite powder in the powder mixture can be consistently prevented. At the same time, agglutination of the powder mixture by electrification of the polyolefin wax can be prevented by addition of the negatively-charged powder to the mixed powder yielding the high flowability.

The disclosure of the present application relates to the main subject described in Japanese Patent Application No. 2013-28703 applied on Feb. 18, 2013, and the content of such disclosure thereof is incorporated herein by references.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating an example of a powder mixture of the present invention.

FIG. 2 is a schematic view illustrating the state of the electrically charged powder mixture. FIG. 2( a) shows the case in which the powder mixture is only a powder in which the graphite powder 2 and the auxiliary raw material powder 3 are adhered to the surface of the main raw material powder 1 with the binding agent 4, and FIG. 2( b) shows the case in which a negatively-charged powder is added to the powder in which the graphite powder 2 and the auxiliary raw material powder 3 are adhered to the surface of the main raw material powder 1 with the binding agent 4.

FIG. 3 is a schematic view illustrating the state of powder prepared by the melting and mixing method.

FIG. 4 is a view illustrating the outline of a compacting machine in the compacting process by the die pressing method.

DESCRIPTION OF EMBODIMENTS

In the present invention, since the powder mixture is a mixture in which a graphite powder is adhered to the surface of the main raw material powder by using the binding agent containing a polyolefin wax with excellent adhering properties, segregation of the graphite powder in the powder mixture can be consistently prevented. At the same time, agglutination of the powder mixture by electrification of the polyolefin wax can be prevented by addition of the negatively-charged powder to the mixed powder yielding the high flowability.

In the present invention, since a general type of materials is used as the polyolefin wax or a coating material for the treatment for negatively charging respectively, its operation is easy. Also, in the present invention, the effect of preventing segregation above and high flowability can be consistently obtained as illustrated in Tables 1 and 3.

Further in the aspect comprising an auxiliary raw material powder in the present invention, the specification in which all or parts of the auxiliary raw material powder are adhered to the surface of the main raw material powder together with the graphite powder, the specification in which the rest is contained in a form of free powder, and further the specification in which all of the auxiliary raw material powder are contained in a form of free powder can be valid. Also, in the present invention, the aspect comprising a lubricant powder for compacting has the specification in which the lubricant powder for compacting is contained in a form of free powder.

In the present invention the negatively-charged powder is to be mixed with the main raw material powder, and consists of the iron powder and/or the iron-based alloy powder treated for negatively charging. In an aspect comprising the auxiliary raw material powder such an aspect can be accepted as parts of the auxiliary raw material powder are treated for negatively charging and the negatively-charged powder consists of the auxiliary raw material powder treated for negatively charging instead of the iron powder and/or the iron-based alloy powder treated for negatively charging, and such an aspect can be accepted as the negatively-charged powder further contains the auxiliary raw material powder treated for negatively charging in addition to the iron powder and/or the iron-based alloy powder treated for negatively charging.

These aspects demonstrate a detail of each invention above for confirmation.

Hereinafter, after an optimal mode of the present invention will be described, its usefulness will be demonstrated using examples.

The schematic view in FIG. 1 illustrates an example of the structure of a powder mixture in the present invention. The powder mixture in this figure comprises main raw material powder 1 consisting of an iron powder or an iron-based alloy powder, graphite powder 2, auxiliary raw material powder 3 consisting of copper powder, lubricant powder for compacting 6, negatively-charged powder 5, and binding agent 4.

The graphite powder 2 and the auxiliary raw material powder 3 are adhered to the surface of the main raw material powder 1 with the binding agent 4 containing the polyolefin wax. The negatively-charged powder 5 and the lubricant powder for compacting 6 are not binded to the main raw material powder 1, but exist in a form of free powder.

The binding agent 4 herein as the first required property needs to keep graphite powder adhered to the main raw material powder from the conveying stage to the compacting stage. Therefore, it becomes important for the binding agent to have not only a high adhering force but also strength high enough for resisting against the vibration when conveyed. For example, the binding agent (lubricant) in the patent literature 1 is brittle, and while it can temporarily adhere graphite powder to the surface of the main raw material powder, it is easily delaminated by vibration when conveyed so that segregation of the graphite powder cannot be prevented.

The polyolefin wax used as the binding agent in the present invention has a high adhering force as well as a certain level of ductility, and has strength high enough for resisting against the vibration when conveyed.

The binding agent as the second required property needs to be easily decomposed when heated for sintering in sintering of a compact and to have no adverse effect on the sintered compact. Also, since the binding agent is a substance to be disappeared in this way, it is desired to be inexpensive as much as possible. In this point, the polyolefin wax used as the binding agent in the present invention is relatively simple in structure and inexpensive as found from, for example, polyethylene wax and polypropylene wax as well as is easily decomposed on heating to disappear, and has no adverse effect on the sintered compact.

The binding agent as the third required property needs to possess the property that the graphite powder can be easily adhered to the surface of the main raw material powder. In this point, since the polyolefin wax has a low melting point and can be easily melted, it is possible to perform melt mixing at low temperature to adhere the graphite powder to the surface of the main raw material powder.

The polyolefin wax used as the binding agent increases the adhering properties and strength with an increase of its molecular weight. From this viewpoint, the polyolefin wax with the weight average molecular weight Mw of 1000 or more is preferred. On the other hand, since the melting point and the decomposition temperature are increased with a higher molecular weight, the polyolefin wax with the weight average molecular weight Mw of 400,000 or less is preferably used. From these facts, for example, polyethylene wax (weight average molecular weight Mw is preferably within a range of 1,000 to 40,000, and more preferably within a range of 1,000 to 10,000), polypropylene wax (weight average molecular weight Mw is preferably within a range of 1,000 to 40,000, and more preferably within a range of 10,000 to 35,000) and the like among the polyolefin waxes are preferably used. As polyethylene wax and polypropylene wax two types or more of waxes with the different weight average molecular weight are preferably mixed for use, since the binding agents are not decomposed at once when sintered but decomposed stepwise.

As the polyolefin wax with the above weight average molecular weight, commercially available products can be used or products appropriately prepared by arbitrary production method can be also used.

The binding agent preferably consists of the polyolefin wax.

Since the polyolefin wax used as the binding agent needs to disappear when sintered and to have no adverse effect on the properties of a sintered compact, the amount used is an amount good enough to adhere graphite powder to the surface of the iron powder or the iron-based alloy to be used as the main raw material powder. For example, since the specific gravity of the binding agent is small, its use in a large amount results in an increase of the relative amount of the binding agent contained in the compact so that the density of the compact is reduced thereby and the compactability of the raw material powder is lowered. Therefore, the amount of the polyolefin wax used as the binding agent should be adjusted depending on the additive amount of the graphite powder, and is preferably adjusted for use to 10 to 80 parts by mass relative to 100 parts by mass of the graphite powder as the additive amount.

Incidentally, when the auxiliary raw material powder is adhered to the main raw material powder together with the graphite powder, the amount of the polyolefin wax used is preferably adjusted for use to 10 to 80 parts by mass relative to 100 parts by mass of the graphite powder and the auxiliary raw material powder which are adhered to the main raw material powder.

The additive amount of the graphite powder is preferably from 1 to 2.5 parts by mass relative to 100 parts by mass of the iron powder or the iron-based powder as the main raw material powder. When the powder mixture of the present invention is used for structural part, the additive amount of the graphite powder is preferably from 1 to 1.2 parts by mass relative to 100 parts by mass of the iron powder or the iron-based powder as the main raw material powder.

Preferably the surface of the main raw material powder is completely coated with the polyolefin wax, but it is difficult to obtain such a state and parts of the surface of the main raw material powder are exposed. In the present invention, since the iron powder or the iron-based alloy powder is used as the main raw material powder, parts of the surface of the iron powder or the iron-based alloy powder are exposed.

In the powder of which the surface is coated with the polyolefin wax for adhering the graphite powder, the surface of the polyolefin wax is negatively charged by friction when conveyed. Also, since the polyolefin wax has the high insulation property, the state of the surface of the polyolefin wax becomes negatively charged. On the other hand, since the iron powder or the iron-based alloy powder of which parts of the surface are exposed is positively charged by friction when conveyed, an electrically attractive force exerts on the surrounding powder coated with the polyolefin wax which is negatively charged.

In the present invention, the negatively-charged powder is added in a form of free powder in order to prevent the powder coated with the polyolefin wax from electrically attracting each other. Since the negatively-charged powder is a free powder not-adhered, it is electrically attracted to the exposed part of positively charged iron powder or iron-based alloy powder to cover the exposed part of the iron powder or the iron-based alloy powder. Therefore, the powder coated with the polyolefin wax to which the negatively-charged powder is adsorbed has the whole surface thereof being negatively charged, preventing the powder coated with the polyolefin wax from electrically attracting each other.

Further, excess of negatively-charged powder which is not electrically attracted to the exposed part of the iron powder or the iron-based alloy powder exerts the electrically repulsive force on the powder coated with the polyolefin wax, of which the whole surface is negatively charged, thereby improving the flowability of the powder mixture.

Incidentally, when the surface of the iron powder or the iron-based alloy powder is completely coated with the polyolefin wax, the powder coated with the polyolefin wax above does not electrically attract each other, but even in this case, by addition of the negatively-charged powder, an electrical repulsive force between the negatively-charged powder and the powder coated with polyolefin wax acts and the flowability of the powder mixture can be improved.

In order that the negatively-charged powder expresses the effect to actively attract to the exposed part of the iron powder or the iron-based alloy powder, a powder with a larger negative charge than the polyolefin wax charged by friction is used. For this purpose, an alkylsilane, dialkylsilane, methacrylsilane, alkylhalilde silane, and hexaalkyldisilazane have high negative electrification so that the powder of which the surface is coated with at least one among these compounds is preferably used.

Alkyl groups of the above compounds include alkyl group with carbon numbers 1 to 10, and preferably include methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, octyl group and the like.

The alkylsilane is not specifically limited, but includes methylsilane, ethylsilane, propylsilane, butylsilane, pentylsilane, hexylsilane, octylsilane and the like.

The dialkylsilane is not specifically limited, but includes dimethylsilane, diethylsilane, dipropylsilane and the like.

The methacrylsilane is not specifically limited, but includes organosilane compounds having acryloyloxy group or methacryloyloxy group and the like.

The alkylhalilde silane is not specifically limited, but includes fluoromethylsilane, difluoromethylsilane, chloromethylsilane, fluorochloromethylsilane, fluoroethylsilane and the like.

The hexaalkyldisilazane is not specifically limited, but includes hexamethyldisilazane, hexaethyldisilazane and the like.

Since dimethylsilane, octylsilane, methacrylsilane, fluoroalkylsilane and hexamethyldisilazane have high negative electrification, a powder of which the surface is coated with at least one among these compounds is preferably used as the negatively-charging powder. A powder coated with octylsilane or hexamethyldisilazane is particularly preferable.

The treatment for negatively charging such as coating with the alkylsilane is performed by a mixing process comprising the steps of agitating alkylsilane and the like and the powder to be treated for negatively charging, and elevating temperature to the melting point or higher of the alkylsilane and the like. This mixing process allows coating of the surface of the powder to be treated for negatively charging with the melted alkylsilane and the like followed by lowering temperature to the melting point or less of the alkylsilane and the like to form the solidified film of the alkylsilane and the like on the surface of the powder to be treated for negatively charging.

Incidentally, the negatively-charged powder interfering the sintering and providing an adverse effect on the sintered compact is not preferred. From this point, the powder to be treated for negatively charging above is at least one among an iron powder and an iron-based alloy powder. Since the main raw material powder is the iron powder or the iron-based alloy powder, the powder to be treated for negatively charging consisting of at least one among the iron powder and the iron-based alloy powder is easily diffused together with the main raw material powder mutually to unite as well as does not provide an adverse effect on the structure of the metal in the sintered compact. Use of the powder with the same component as the main raw material powder, as the powder to be treated for negatively charging is herein preferred since there is no adverse effect on the structure of the metal in the sintered compact.

Also, the specific surface area of the negatively-charged powder is increased with a finer size of powder as well as its mass becomes smaller making easier attraction to the exposed part of the positively charged iron powder or the ion-based alloy powder above, that is, the main raw material powder 1. Also, as the negatively-charged powder becomes large in size, it likely disturbs the sintering property.

Therefore, the maximum particle diameter of the negatively-charged powder is preferably 15 μm or less. On the other hand, as the particle diameter becomes too small, the negatively-charged powder added in a form of powder is likely penetrated into the space between the die and the punch so that scuffing of the die easily occurs. Therefore, the minimum particle diameter of the negatively-charged powder is preferably 1 μm or more.

The average particle diameter (D50) of the main raw material powder is preferably within the range from 50 to 150 μm, and more preferably within the range from 50 to 120 μm. The average particle diameter (D50) of the auxiliary raw material powder is preferably within the range from 50 to 100 μm, and also smaller than that of the main raw material powder. The average particle diameter (D50) of the primary particle diameter of the graphite powder is preferably within the range from 1 to 50 μm, and more preferably within the range from 1 to 10 μm.

As the main raw material powder, the auxiliary raw material powder, and the graphite powder with the above particle diameter respectively, commercially available products can be used, or products obtained by preparing by arbitrary production method and then selecting with sieve or the like can be also used.

Further, the additive amount of the negatively-charged powder is preferably from 0.02 to 0.5 parts by mass relative to 100 parts by mass of the iron powder or the iron-based alloy powder (main raw material powder). This is due to the fact that as the additive amount of the negatively-charged powder is below 0.02 parts by mass, it is difficult to obtain its effect consistently, and also as the additive amount is above 0.5 parts by mass, the ejection pressure of a pressurized powder-compact is substantially increased after compacting the powder.

The auxiliary raw material powder is a powder having the effect of solid solution strengthening of the base of the sintered compact by diffusing into the base of the sintered compact formed with the main raw material powder, the effect of improving the strength of the sintered compact by forming a compound to reinforce the base of the sintered compact, the effect of improving the properties such as hardenability of the base of the sintered compact by diffusing into the base of the sintered compact formed with the main raw material powder, the effect of improving the strength of the sintered compact by activating and promoting the sintering, and the effect of improving the properties such as wear resistance of the sintered compact and machinability of the sintered compact by dispersing into the sintered compact.

Also, when the iron powder or the iron-based alloy powder is used as the main raw material powder, as the auxiliary raw material powder, for example, graphite powder, copper powder or copper alloy powder such as copper-tin alloy powder, nickel powder, molybdenum powder, iron-based alloy powder (in this case a small amount of the iron-based alloy powder with a different component from the main raw material powder) such as iron-phosphorous alloy powder and the like, various hard phase-forming powders, magnesium silicate type mineral powder, calcium fluoride powder, sulfide powder, and the like are used. The amount of the auxiliary raw material powder used is 30 parts by mass or less, preferably 20 parts by mass or less, and further preferably 10 parts by mass or less relative to 100 parts by mass of the main raw material powder.

Such an auxiliary raw material powder is preferably adhered to the surface of the iron powder or the iron-based alloy powder similarly to graphite powder as the auxiliary raw material powder 3 in FIG. 1. In particular, when the auxiliary raw material powder with the lower specific gravity such as the exemplified magnesium silicate type mineral powder, calcium fluoride powder, and sulfide powder in part as compared to the iron powder or the iron-based alloy powder to be the main raw material powder are used, or when the powder with a smaller particle diameter is used as the auxiliary raw material powder, segregation of the powder is likely to occur similarly to graphite powder so that the auxiliary raw material powder are preferably adhered with the graphite powder to the surface of the iron powder or the iron-based alloy powder to be used as the main raw material powder.

On the other hand, when the powder with the close specific gravity to the iron powder or the iron-based alloy powder and having a certain level of particle size is used as the auxiliary raw material powder, segregation of the powder is unlikely to occur so that the powder can be added in a form of free powder. In this case, since the auxiliary raw material powder is not required to be adhered, the amount of polyolefin wax can be reduced by the amount for the auxiliary raw material powder.

Therefore, when the auxiliary raw material powder consists of a plural kind of powders and parts of the powder are likely to segregate, it is preferred to adhere only parts of the auxiliary raw material powders which are likely to segregate to the surface of the iron powder or the iron-based alloy powder and use the rest of the auxiliary raw material powders which resist segregation in a form of free powder, since the amount of the polyolefin wax used is reduced to only the requirement.

When the auxiliary raw material powder is used as described above, the auxiliary raw material powder treated for negatively charging can be used as the negatively-charged powder instead of, or in addition to at least one among the iron powder and the iron-based alloy powder used as the negatively-charged powder. This is due to the fact that when the auxiliary raw material powder is used as a negatively-charged powder, it is easily diffused together with the main raw material powder to unite as well as it does not provide an adverse effect on the structure of the metal in the sintered compact.

The polyolefin wax is generally used as the lubricant, but in the powder mixture of the present invention the polyolefin wax is used as the binding agent so that its function as the lubricant for compacting is low. Therefore, when a die wall lubrication method (external lubrication method) is used, the method comprising the steps of applying a powder lubricant or a liquid lubricant to the surface of the die wall and then performing the compacting, the powder mixture can be used as it is. On the other hand, when the die is used without applying the powder lubricant or the liquid lubricant to the surface of the die wall for the compacting, a mixed lubrication method (internal lubrication method) in which the lubricant powder for compacting is mixed with the raw material powder is preferably used.

As the lubricant powder for compacting, the material conventionally used can be used. For example, a fatty acid such as stearic acid and the like, and a metal salt of the fatty acid such as zinc stearate, lithium stearate and the like can be used. Since the property of lubrication is reduced when provided in a melted form, it is preferably provided in a form of free powder. Also, the additive amount of the lubricant powder for compacting is within the range from 0.1 to 1.5 parts by mass relative to 100 parts by mass of the main raw material powder as conventionally used.

A material conventionally used in a powder metallurgy process can be used as the lubricant powder for compacting, and the particle diameter and the like thereof are not particularly limited.

The powder mixture above can be manufactured, for example, as follows. That is, the manufacture method is a method comprising a process of adhering the graphite powder, or the graphite powder and the auxiliary raw material powder to the surface of the iron powder or the iron-based alloy powder through the binding agent containing the polyolefin wax, and a process of adding the negatively-charged powder consisting of the iron powder and/or the iron-based alloy powder treated for negatively charging to the raw material powder obtained in the process above and mixing them.

As described in detail, firstly a primary mixing process of elevating temperature to the melting point or higher of the polyolefin wax while agitating a mixture of the binding agent containing the polyolefin wax and the iron powder or the iron-based alloy powder, is performed. The surface of the iron powder or the iron-based alloy powder is coated with the melted polyethylene wax by the primary mixing process.

Next a secondary mixing process of adding graphite powder to the powder mixture obtained in the primary mixing process at the temperature of the melting point or higher of the polyethylene wax and agitating them, is performed. The graphite powder is attached by the secondary mixing process to the melted polyolefin wax which coats the surface of the iron powder or the iron-based alloy powder. The mixture is cooled from this state to the melting point or lower of the polyolefin wax, thereby yielding the powder mixture in which the graphite powder is adhered by the binding agent to the surface of the iron powder or the iron-based alloy. The secondary process can be performed such that after the primary mixing process, the mixture is once cooled to form the powder mixture, and then the powder mixture is reheated separately, but considering the time required for cooling and energy loss for reheating, the secondary mixing process is preferably performed such that the graphite powder is successively added without cooling the mixture after the primary mixing process.

After the secondary mixing process a tertiary mixing process of adding the negatively-charged powder to the powder mixture obtained at the temperature of the melting point or higher of the polyolefin wax and mixing them, is performed. The negatively-charged powder can be provided in a form of free powder by performing the tertiary mixing process at the temperature of the melting point or lower of the polyolefin wax. The tertiary mixing process can be initiated when the temperature becomes the melting point or lower of the polyolefin wax in the cooling step after the secondary mixing process, but the negatively-charged powder can be added and mixed at the ambient temperature after cooling the powder mixture obtained in the secondary mixing process to the ambient temperature. When the lubricant powder for compacting is added, the lubricant powder for compacting can be provided in a form of free powder by adding and mixing the lubricant powder for compacting in the tertiary mixing process.

When the powder mixture comprises the auxiliary raw material powder, the auxiliary raw material powder can be adhered to the surface of the iron powder or the iron-based alloy powder by adding and mixing the auxiliary raw material powder in the secondary mixing process. Also, as the auxiliary raw material powder is added and mixed in the tertiary mixing process, the auxiliary raw material powder can be provided in a form of free powder.

EXAMPLES Example 1

Iron powder (100 mesh, average particle diameter (D50): 75 μm), electrolytic copper powder (200 mesh, average particle diameter (D50): 45 μm), graphite powder (325 mesh, average particle diameter (D50): 10 μm), and zinc stearate powder as the lubricant powder for compacting were prepared as well as a polyolefin wax (polyethylene wax) with the weight average molecular weight of 8,000 was prepared. Iron powder of which the surface was coated with an alkylsilane (octylsilane) and iron powder of which the surface was coated with hexamethyldisilazane (maximum particle diameter of each powder is 5 μm) were prepared as the negatively-charged powders.

The primary mixing process was performed by adding 0.5 parts by mass of the polyolefin wax relative to 100 parts by mass of the iron powder, feeding them into a Henschel mixer, and mixing them in the mixer while elevating temperature to 130° C. higher than the melting point of the polyolefin wax (110° C.) to coat the surface of iron powder with the melted polyolefin wax. Next the secondary mixing process was performed by adding a copper powder and a graphite powder to the mixture while keeping the melted state of the polyolefin wax so as to adjust the amount of the copper powder and the graphite powder to 1.5 parts by mass and 1.0 part by mass, respectively, relative to 100 parts by mass of the iron powder, and mixing them to fully attach the copper powder and the graphite powder to the iron powder by the melted polyolefin wax and to uniformly disperse the powders. Thereafter, the mixture was cooled to the ambient temperature while agitating to yield a secondary mixture. The secondary mixture is a product in which 1.5 parts by mass of the copper powder and 1.0 part by mass of the graphite powder are adhered with 0.5 parts by mass of the polyolefin wax as a binding agent relative to 100 parts by mass of the iron powder. The additive amount of the polyolefin wax is 50 parts by mass relative to 100 parts of the graphite powder added.

A negatively-charged powder was added to the secondary mixture obtained in a ratio (the addition rate is based on the part by mass relative to 100 parts of iron powder) indicated in Table 1 as well as 0.8 parts by mass of the lubricant powder for compacting was added to the mixture, which was mixed in a V-shape mixer to prepare the powder mixtures of sample numbers of 01 to 17. For comparison, 1.5 parts by mass of the copper powder, 1.0 part by mass of the graphite powder, and 0.8 parts by mass of the lubricant powder for compacting were added relative to 100 parts by mass of the iron powder, which were mixed in the V-shape mixer to prepare the powder mixture of sample number 18.

The amount of graphite powder attached, the flow rate as flowability, and the ejection pressure (ejection pressure, MPa) of these powder mixtures were measured.

Among these measurements, firstly to determine the attachment rate, the carbon content of a powder mixture (sample number 01) in which the negatively-charged powder was not added, was determined by the carbon content analysis using the infrared absorption method after combustion in a high frequency induction furnace defined in G1211 of JIS Standard. The measured carbon content is a carbon content of the whole sample of the powder mixture including the graphite powder not adhered but free (the carbon content of a total of the carbon component in graphite powder and wax). Next, to prevent effects of the graphite powder which could not be adhered but free and negatively-charged powder which was added in a form of free powder, the mixed powder was classified by a 100 mesh sieve and a 200 mesh sieve to collect the powder which was passed through the 100 mesh sieve but not through the 200 mesh sieve (powder with the particle diameter of 75 to 150 μm), and the carbon content of the powder collected was determined by the carbon content analysis using the infrared absorption method after combustion in a high frequency induction furnace defined in G1211 of JIS Standard. The measured carbon content is a carbon content of the graphite powder attached to the iron powder through the wax (the carbon content of a total of the carbon component in graphite powder and wax). The attachment rate of graphite powder is determined as the ratio of the carbon content in graphite powder attached to iron powder through wax to the carbon content of a whole sample of the powder mixture including the free graphite powder by using the measured carbon content of a whole sample of the powder mixture and the measured carbon content of the graphite powder attached to the carbon powder through wax.

Measurement of the flow rate of the powder mixture was performed by using the method for determination of flow rate defined in Z2502 of JIS Standard.

To determine the ejection pressure, a die set of a floating die type in which the die was supported by a spring and a lower punch, and having the structure of which a load cell was incorporated into the pressure receiving plate of the lower punch was used, a cylindrical compact with a diameter of 11.3 mm and a height of 10 mm was compacted at pressure of 700 MPa by using an Amsler type universal test machine, the ejection force when ejecting the compact from the die was measured, and the ejection pressure was calculated by dividing the load pressure measured with the outer circumferential area of the cylindrical compact. The values are also shown in Table 1.

TABLE 1 Negatively-charged powder Iron powder Iron powder coated with Attachment Flow Ejection Sample coated with hexamethyl- rate rate pressure No. alkylsilane disilazane (%) (Sec.) (MPa) 01 0.0 97 36 12.8 02 0.02 97 32 12.9 03 0.05 97 30 12.9 04 0.1 97 28 13.1 05 0.2 97 27 13.3 06 0.3 97 26 13.4 07 0.4 97 26 13.9 08 0.5 97 26 14.2 09 1.0 97 26 16.2 01 0.0 97 36 12.8 10 0.02 97 31 12.8 11 0.05 97 29 12.9 12 0.1 97 28 12.9 13 0.2 97 27 13.2 14 0.3 97 26 13.3 15 0.4 97 26 13.8 16 0.5 97 26 14.3 17 1.0 97 26 16.2 18 — — 30 34 18.1

Table 1 shows the results in which effects of the additive amount of the negatively-charged powder were studied, and sample numbers 01 to 09 are examples where the iron powder of which the surface is coated with the alkylsilane (octylsilane) is used and sample numbers 01 and 10 to 17 are examples where the iron powder of which the surface is coated with hexamethyldisilazane is used.

Sample number 18 in Table 1 is an example of the powder mixture in which the powder is simply mixed conventionally without use of the polyolefin wax. The attachment rate of the graphite powder is 30%, low in value in the powder which is passed through a 100 mesh sieve but not through a 200 mesh sieve (powder with the particle diameter within the range of 75 to 150 μm). Incidentally, the graphite powder with the attachment rate of 30% is a graphite powder which is trapped in the depressed area of irregular shaped iron powder. On the other hand, the powder mixture sample of sample number 01 is a sample in which the graphite powder is attached to the surface of the iron powder by the polyolefin wax, and indicating that the attachment rate of graphite powder is 97%, high in attachment rate. However, the flow rate in the powder mixture sample of sample number 01 is reduced relative to the flow rate in sample number 18 in which powders are simply mixed conventionally to form a powder mixture.

Powder mixture samples (sample numbers 02 and 10) in which 0.02 parts by mass of the negatively-charged powder is added to such the powder mixture in which the graphite powder is attached to the surface of the iron powder by the polyolefin wax improve the flowability of the powder mixture and have a smaller flow rate as compared to the powder mixture of sample number 18 in which the powder is simply mixed conventionally. Also, the more the additive amount of the negatively-charged powder increases, the more the flowability of the powder mixture is improved and the smaller the flow rate is. However, further improvement in the flow rate is not observed even when the additive amount of the negatively-charged powder to the powder mixture exceeds 0.3 parts by mass.

It is confirmed from the above results that the attachment rate of graphite powder can be substantially improved by attaching graphite powder to the surface of the iron powder using the polyolefin wax but the flowability is reduced, and the reduction of the flowability can be improved by adding 0.02 parts by mass or more of the negatively-charged powder relative to 100 parts by mass of the iron powder to the powder mixture in which the graphite powder is attached to the surface of the iron powder using the polyolefin wax, and in this case its flowability can be further improved as compared to the powder mixture in which the powder is simply mixed conventionally.

It is also confirmed that the effects of the negatively-charged powder above can be similarly obtained when either iron powder coated with the alkylsilane or iron powder coated with hexamethyldisilazane is used as the negatively-charged powder, and it is confirmed that similar effects can be obtained as long as a negatively-charged powder is used.

Incidentally, in the powder mixture (sample number 01) in which the graphite powder is attached to the surface of the iron powder by the polyolefin wax, the ejection pressure is reduced to a range of 70% of the ejection pressure of the powder mixture (sample number 18) in which the powder is simply mixed conventionally, because the wax works as a lubricant. As the negatively-charged powder is added to the powder mixture in which the graphite powder is attached to the surface of the iron powder by the polyolefin wax, the ejection pressure is increased. It is also found that the ejection pressure tends to increase with an increase of the additive amount of negatively-charged powder, and in the samples (sample numbers 08 and 16) of the powder mixture in which the additive amount of negatively-charged powder is 0.5 parts by mass, the ejection pressure is reduced to a range of 80% of the ejection pressure of the powder mixture (sample number 18) in which the powder is simply mixed conventionally. As the additive amount of negatively-charged powder exceeds 0.5 parts by mass (sample number 17), the ejection pressure is reduced to a range of 90% of the ejection pressure of the powder mixture (sample number 18) in which the powder is simply mixed conventionally. It is found from the results above that the additive amount of the negatively-charged powder to the powder mixture in which the graphite powder is attached to the surface of iron powder by the polyolefin wax is preferably 0.5 parts by mass or less.

Example 2

In Example 2 the powder mixtures of sample numbers of 19 to 29 were prepared by using the iron powder, the copper powder, the graphite powder, and the lubricant powder for compacting used in Example 1 above as well as using the iron powder coated with an alkylsilane (octylsilane) in the same manner in Example 1 except that the polyolefin wax was changed to a wax (polyethylene wax) with the weight average molecular weight indicated in Table 2. The attachment rate, the flow rate, and the ejection pressure of the powder mixture obtained were measured similarly to Example 1. The results are shown in Table 2.

TABLE 2 Weight average Attachment Flow Ejection Sample molecular weight rate rate pressure No. of polyolefin wax (%) (Sec.) (MPa) 19 500 94 34 12.2 20 1,000 95 30 12.8 21 3,000 95 29 12.9 22 5,000 97 28 13.0 05 8,000 97 27 13.3 23 10,000 97 25 13.6 24 15,000 97 25 14.7 25 20,000 97 25 16.2 26 25,000 97 25 16.5 27 30,000 97 25 17.0 28 35,000 97 25 17.0 29 40,000 97 25 17.6 18 — 30 34 18.1

Table 2 indicates that the powder mixture in which the graphite powder is attached to the surface of the iron powder by using the polyolefin wax has the higher attachment rate of the graphite powder as compared to the powder mixture (sample number 18) in which the powder is simply mixed conventionally, regardless of the weight average molecular weight Mw of the polyolefin wax.

Incidentally, in the powder mixture sample (sample number 19) in which the weight average molecular weight Mw of the polyolefin wax is 500, the wax coating the surface of the iron powder is soft so that the attachment rate is somewhat lower and the flow rate is in the same level as the powder mixture (sample number 18) in which the powder is simply mixed conventionally. On the other hand, in the powder mixture samples (sample numbers 05 and 20 to 29) in which the weight average molecular weight Mw of the polyolefin wax is 1000 or more, the waxes coating the surface of the iron powder become harder than the powder mixture sample of sample number 19. Therefore, the attachment rate is improved and the flowability is improved in each sample as compared to that of the powder mixture (sample number 18) in which the powder is simply mixed conventionally. It is found from these results that use of the polyolefin wax with the weight average molecular weight Mw of 1000 or more is desired, since not only the attachment rate of the graphite powder but also the flowability can be improved.

On the other hand, the ejection pressure tends to increase with an increase of the weight average molecular weight Mw of the polyolefin wax. In the powder mixture sample (sample number 29) in which the weight average molecular weight Mw of the polyolefin wax is 40,000, the ejection pressure is still lower in value than that of the powder mixture (sample number 18) in which the powder is simply mixed conventionally. However, as the weight average molecular weight Mw of the polyolefin wax exceeds 40,000, it is considered that the ejection pressure increases to the same level to that of the powder mixture (sample number 18) in which the powder is simply mixed conventionally. Therefore, considering the ejection pressure, the polyolefin wax with the weight average molecular weight Mw of 40,000 or less is preferably used.

Example 3

In Example 3 the powder mixtures of sample numbers of 30 to 37 were prepared by using the iron powder, the copper powder, the graphite powder and the lubricant powder for compacting used in Example 1 in the same manner in Example 1 except for using copper powder (maximum particle diameter: 5 μm) coated with an alkylsilane (octylsilane) as the negatively-charged powder and except that the blending ratio was followed as indicated in Table 3. The attachment rate, the flow rate, and the ejection pressure of the powder mixture obtained were measured similarly to Example 1. The results are shown in Table 3.

TABLE 3 Blending ratio mass part Lubricant Copper powder Attachment Flow Ejection Sample Iron Polyolefin Copper Graphite powder for coated with rate rate pressure No. powder wax powder powder compacting alkylsilane (%) (Sec.) (MPa) 01 100 0.5 1.5 1.0 0.8 0.0 97 36 12.8 30 100 0.5 1.5 1.0 0.8 0.02 97 36 12.9 31 100 0.5 1.5 1.0 0.8 0.05 97 34 13.0 32 100 0.5 1.4 1.0 0.8 0.1 97 33 13.1 33 100 0.5 1.3 1.0 0.8 0.2 97 32 13.2 34 100 0.5 1.2 1.0 0.8 0.3 97 30 13.4 35 100 0.5 1.1 1.0 0.8 0.4 97 29 13.9 36 100 0.5 1.0 1.0 0.8 0.5 97 29 14.3 37 100 0.5 0.5 1.0 0.8 1.0 97 28 16.3 18 100 — 1.5 1.0 0.8 — 30 34 18.1

Table 3 shows the results in which the effects are studied when the auxiliary raw material powder is used as the negatively-charged powder, and sample numbers 30 to 37 are the examples in which copper powder of which the surface is coated with an alkylsilane (octylsilane) is used instead of iron powder treated for negatively charging of Example 1.

Table 3 indicates that in the powder mixture sample (sample number 30) in which 0.02 parts by mass of the negatively-charged powder using the auxiliary raw material powder is added to the powder mixture in which the graphite powder is attached to the surface of the iron powder by the polyolefin wax (polyethylene wax), the flowability of the powder mixture is improved and the flow rate becomes smaller as compared to the powder mixture of sample number 18 in which the powder is simply mixed conventionally. Also, the flowability of the powder mixture is further improved with an increase of the additive amount of the negatively-charged powder and the flow rate is reduced. However, further improvement in the flowability is not observed even when the additive amount of the negatively-charged powder to a powder mixture exceeds 0.3 parts by mass.

Also, addition of the negatively-charged powder using the auxiliary raw material powder to the powder mixture in which the graphite powder is attached to the surface of iron powder by the polyolefin wax increases the ejection pressure. It is also found that the ejection pressure tends to increase with an increase of the additive amount of the negatively-charged powder. In the powder mixture sample (sample number 36) in which the additive amount of the negatively-charged powder is 0.5 parts by mass, the ejection pressure is reduced to a range of 80% of the ejection pressure of the powder mixture (sample number 18) in which the powder is simply mixed conventionally, and as the additive amount of the negatively-charged powder exceeds 0.5 parts by mass (sample number 37), the ejection pressure is reduced to a range of 90% of the ejection pressure of the powder mixture (sample number 18) in which the powder is simply mixed conventionally.

It is found from the results above that as iron powder (similar component of main raw material powder) is replaced with copper powder (component of auxiliary raw material powder) for the powder to be treated for negatively charging, similar results can be obtained.

INDUSTRIAL APPLICABILITY

Since the powder mixture of the present invention is a powder mixture in which the segregation of the powder in the mixed powder is prevented and the flowability of the mixed powder is excellent and a sintered machine part can be manufactured by the die pressing method without variation in quality, it is suitable for manufacture of various sintered machine parts.

EXPLANATION OF REFERENCES

-   1 main raw material powder -   2 graphite powder -   3 auxiliary raw material powder -   4 binding agent -   5 negatively-charged powder -   6 lubricant powder for compacting -   10 die -   11 die hole -   20 lower punch -   30 upper punch -   40 hopper -   50 hose -   60 feeder 

1. A powder mixture comprising: an iron powder or an iron-based alloy powder, wherein a graphite powder is adhered to a surface of the iron powder or the iron-based alloy powder by a binding agent containing a polyolefin wax; and a negatively-charged powder to be mixed with the iron powder or the iron-based alloy powder, and consisting of an iron powder and/or an iron-based alloy powder treated for negatively charging.
 2. The powder mixture according to claim 1, wherein the polyolefin wax is at least one selected from the group consisting of polyethylene wax and polypropylene wax.
 3. The powder mixture according to claim 1, wherein the treatment for negatively charging is coating treatment with at least one selected from the group consisting of an alkylsilane, dialkylsilane, methacrylsilane, alkylhalide silane, and hexaalkyldisilazane.
 4. The powder mixture according to claim 1, wherein the maximum particle diameter of the negatively-charged powder is within the range from 1 to 15 μm.
 5. The powder mixture according claim 1, wherein an additive amount of the negatively-charged powder is from 0.02 to 0.5 parts by mass relative to 100 parts by mass of the iron powder or the iron-based alloy powder.
 6. The powder mixture according to claim 1, wherein the powder mixture further comprises an auxiliary raw material powder, and the auxiliary raw material powder is adhered together with the graphite powder by the binding agent to the surface of the iron powder or the iron-based alloy powder.
 7. The powder mixture according to claim 1, wherein the powder mixture further comprises an auxiliary raw material powder, and the auxiliary raw material powder is contained in a form as being adhered together with the graphite powder by the binding agent to the surface of the iron powder or the iron-based alloy powder as well as in a form of free powder.
 8. The powder mixture according to claim 1, wherein the powder mixture further comprises an auxiliary raw material powder, and the auxiliary raw material powder is contained in a form of free powder.
 9. The powder mixture according to claim 6, wherein a part of the auxiliary raw material powder is an auxiliary raw material powder treated for negatively charging, and the negatively-charged powder consists of the auxiliary raw material powder treated for negatively charging instead of the iron powder and/or the iron-based alloy powder treated for negatively charging.
 10. The powder mixture according to claim 6, wherein a part of the auxiliary raw material powder is an auxiliary raw material powder treated for negatively charging, and the negatively-charged powder further contains the auxiliary raw material powder treated for negatively charging in addition to the iron powder and/or the iron-based alloy powder treated for negatively charging.
 11. The powder mixture according to claim 1, wherein the powder mixture further comprises a lubricant powder for compacting, and the lubricant powder for compacting is contained in a form of free powder.
 12. The powder mixture according to claim 7, wherein a part of the auxiliary raw material powder is an auxiliary raw material powder treated for negatively charging, and the negatively-charged powder consists of the auxiliary raw material powder treated for negatively charging instead of the iron powder and/or the iron-based alloy powder treated for negatively charging.
 13. The powder mixture according to claim 8, wherein a part of the auxiliary raw material powder is an auxiliary raw material powder treated for negatively charging, and the negatively-charged powder consists of the auxiliary raw material powder treated for negatively charging instead of the iron powder and/or the iron-based alloy powder treated for negatively charging.
 14. The powder mixture according to claim 7, wherein a part of the auxiliary raw material powder is an auxiliary raw material powder treated for negatively charging, and the negatively-charged powder further contains the auxiliary raw material powder treated for negatively charging in addition to the iron powder and/or the iron-based alloy powder treated for negatively charging.
 15. The powder mixture according to claim 8, wherein a part of the auxiliary raw material powder is an auxiliary raw material powder treated for negatively charging, and the negatively-charged powder further contains the auxiliary raw material powder treated for negatively charging in addition to the iron powder and/or the iron-based alloy powder treated for negatively charging. 